The former president of Bankia, Rodrigo Rato, and 32 former members of the failed bank's board are to be investigated on charges of falsifying accounts, dishonest administration, price manipulation and improper appropriation.

Bankia was nationalised in May at a cost of €23bn (£18bn) to the public purse after its share price fell by 75% in less than a year.

The investigation comes in response to demands from the tiny Unión Progresso y Democracia party, whose leader Rosa Díez accused Bankia of carrying out "a massive fraud" on its small investors because, in her opinion, its stock market launch was based on "the falsification of data".

The Audiencia Nacional, the court that deals with serious crime, decided to investigate after the anti-corruption prosecutor's office said it believed there was a case to answer.

The judge has also summoned the former governor of the Bank of Spain, the president of Stock Market National Commission, the Deloitte partner who worked for Bankia and the legal representative of the Frob (Fondo de Reestructuración Ordenada Bancaria) to testify.

The Audiencia Nacional has also demanded from the savings banks that comprise the BFA-Bankia group details of all credits, loans and guarantees made since 2008 in favour of the board and executives and their families, as well political parties.

Rato was the Spanish finance minister from 1996 to 2004. After his Popular party lost the election in 2004 he went on to head the International Monetary Fund. Another of the accused, Ángel Acebes, was interior minister in the same administration.